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Implementing PREMIS in Container Formats

This article discusses the implementation of PREMIS (Preservation Metadata: Implementation Strategies) in container formats for digital preservation. It outlines the challenges and strategies for encoding, storing, managing, and exchanging preservation metadata. The article also provides a comprehensive view of the PREMIS Data Model and guidelines for its implementation.

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Implementing PREMIS in Container Formats

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  1. Implementing PREMIS in Container Formats Rebecca Guenther, Library of Congress rgue@loc.goc Zhiwu Xie, Los Alamos National Laboratory zxie@lanl.gov IS&T’s Archiving 2007 Arlington, VA, May 23, 2007

  2. OUTLINE • Introduction • OAIS Reference Model and Containers • METS • MPEG-21 DID • Implementing PREMIS in METS • Implementing PREMIS in MPEG-21 DID • Summary

  3. Digital preservation: imperative and challenge • More and more of scholarly and cultural record exists in digital form; steps must be taken to secure its long-term future • Significant progress has been made in raising awareness about digital preservation imperative • Shift in focus from articulatingproblem to solvingit… • Not so much “Why is digital preservation important”, but “What must be done to achieve preservation objectives?” • Many practical challenges in implementing reliable, sustainable digital preservation programs • One key challenge: preservation metadata

  4. PREMIS Working Group • June 2003: OCLC, RLG sponsored new international working group: • PREMIS: Preservation Metadata: Implementation Strategies • Membership: • > 30 experts from 5 countries, representing libraries, museums, archives, government agencies, and the private sector • Co-Chairs: Priscilla Caplan (FCLA), Rebecca Guenther (LC) • Objective 1: Identify and evaluate alternative strategies for encoding, storing, managing, and exchanging preservation metadata • PREMIS Survey Report (September 2004) • Snapshot of current practices/emerging trends related to managing and using preservation metadata in digital archiving systems • http://www.oclc.org/research/projects/pmwg/surveyreport.pdf • Objective 2: Define implementable, core preservation metadata, with guidelines/recommendations for management and use

  5. PREMIS Data Model Intellectual Entities Rights Agents Objects Events

  6. PREMIS Data Dictionary • May 2005: Data Dictionary for Preservation Metadata: Final Report of the PREMIS Working Group • 237-page report includes: • PREMIS Data Dictionary 1.0 • Context/assumptions, data model, usage examples • Set of XML schema to support implementation • Data Dictionary: • Comprehensive view of information needed to support digital preservation • Guidelines/recommendations to support creation, use, management • Used Framework as starting point • Based on deep pool of institutional experiences in setting up and managing operational capacity for digital preservation • Received the 2005 Digital Preservation Award (UK) and 2006 Society of American Archivists Publication Award http://www.oclc.org/research/projects/pmwg/premis-final.pdf

  7. Some guiding principles … • “Implementable, core, preservation metadata”: • “Preservation metadata”: maintain viability, renderability, understandability, authenticity, identity in a preservation context • “Core”: What most preservation repositories need to know to preserve digital materials over the long-term • “Implementable”: rigorously defined; supported by usage guidelines/recommendations; emphasis on automated workflows • “Technical neutrality”: • Digital archiving system: no assumptions about specific archiving technology, system/DB architectures, preservation strategy • Metadata management: no assumptions about whether metadata is stored locally or in external registry; recorded explicitly or known implicitly; instantiated in one metadata element or multiple elements • Promotes flexibility, applicability in wide range of contexts

  8. Scope • What PREMIS DD is: • Common data model for organizing/thinking about preservation metadata • Guidance for local implementations • Standard for exchanging information packages between repositories • What PREMIS DD is not: • Out-of-the-box solution: need to instantiate as metadata elements in repository system • All needed metadata: excludes business rules, format-specific technical metadata, descriptive metadata for access, non-core preservation metadata • Lifecycle management of objects outside repository • Rights management: limited to permissions regarding actions taken within repository

  9. PREMIS Maintenance Activity • Web site: • Permanent Web presence, hosted by Library of Congress • Central destination for PREMIS-related info, announcements, resources • Home of the PREMIS Implementers’ Group (PIG) discussion list • PREMIS Editorial Committee: • Set directions/priorities for PREMIS development • Coordinate future revisions of Data Dictionary and XML schema • Membership: Library of Congress, OCLC, FCLA, National Archives of Scotland, British Library, National Library of Australia, U. of Goettingen, LANL, Ex Libris, Library Archives Canada • http://www.loc.gov/standards/premis/

  10. OAIS Reference Model and Containers

  11. OAIS Reference Model • Developed by the Consultative Committee for Space Data Systems (CCSDS) • ISO 14721:2003 • A functional model for preservation activities • An information model specifying types of information required for long-term preservation

  12. OAIS Reference Model and PREMIS • OAIS reference model specifies the Preservation Description Information (PDI) • PREMIS used the OAIS information model as a starting point • PREMIS Data Dictionary consolidated and further developed the conceptual types of information objects into more than 100 structured and logically integrated semantic units. • PREMIS Data Dictionary provided detailed descriptions and guidelines to implement these semantic units. • PREMIS Data Dictionary does not provide semantic units for Intellectual Entities, but provides semantic units to link to other metadata sources for Intellectual Entities • All entities have reference (identification) information. • No “packaging information” that links content with metadata, but PREMIS can be used with container schemas • PREMIS deals mostly with representation, context, provenance, and fixity information, in keeping with PREMIS definition of preservation metadata.

  13. PREMIS XML schemas • One schema for each PREMIS entity in data model • Allows user to choose which parts of PREMIS to use • PREMIS container schema • References schema for each entity type • Provides a container if it is desirable to keep some or all PREMIS metadata together • If using container requires at least an object which in turn requires objectIdentifier and objectCategory • Individual schemas may used alone or with container • Semantic units in PREMIS schemas • XML is faithful to data dictionary • Only those units mandatory for all categories of objects are mandatory in object schema

  14. Need a Container in the XML Implementation • Archival Information Package (AIP) may include much more metadata besides the preservation metadata • A well defined container is usually necessary to group and appropriately associate these metadata with the data object • For example: METS or MPEG-21 DID

  15. METS records the (possibly hierarchical) structure of digital objects, the names and locations of the files that comprise those objects, and the associated metadata • A METS document may be a unit of storage (e.g. OAIS AIP) or a transmission format (e.g. OAIS SIP or DIP) • METS is extensible and modular • METS uses extension “wrappers” or “sockets” where elements from other schemas can be plugged in • METS uses the XML Schema facility for combining vocabularies from different Namespaces • The METS Editorial Board has endorsed PREMIS as an extension schema • Many institutions trying to use PREMIS within the METS context

  16. The structure of a METS file

  17. OAIS and METS <METS> described by delimited by Descriptive Information Archival Information Package Packaging Information identifies derived from <dmdSec> MODS MARCXML DC Preservation Description Information Content Information further described by <fileGrp> <amdSec> Reference Information <mdRef> Data Object Representation Information <rightsMD> Context Information metsRights premis:rights <file> <techMD> <structMap> <digiProvMD> <sourceMD> premis:event Provenance Information Structure Semantics described by Fixity Information <techMD> premis:object File formats premis:object textMD MIX Legend Black Arial = OAIS Red Times New Roman = METS Primary Schema Blue Times New Roman Italics = Extension Schema

  18. METS extension schemas • “wrappers” or “sockets” where elements from other schemas can be plugged in • Provides extensibility • Uses the XML Schema facility for combining vocabularies from different Namespaces • Endorsed extension schemas: • Descriptive: MODS, DC, MARCXML • Technical metadata: MIX (image); textMD (text) • Preservation related: PREMIS

  19. Issues in using PREMIS with METS • Which METS sections to use and how many • Whether to record elements redundantly in PREMIS that are defined explicitly in the METS schema • How to record elements that are also part of a format specific technical metadata schema (e.g. MIX) • Recording structural relationships • How to deal with locally controlled vocabularies • Whether to use the PREMIS container

  20. PREMIS and METS sections • Flexibility of METS requires implementation decisions • You can’t put all PREMIS metadata directly under amdSec • What sections to use for PREMIS metadata? • Alternative 1 • Object in techMD • Event in digiProvMD • Rights in rightsMD • Agent with event or rights • Alternative 2 • Everything in digiProvMD • Alternative 3 • Everything in techMD • How many administrative MD sections to use? • Experimentation will result in best practices

  21. Inserting technical metadata in a METS Document <mets> <amdSec> <techMD> <mdWrap> <xmlData> <!-- insert data from different namespace here --> </xmlData> </mdWrap> </techMD> </amdSec> <fileSec /> <structMap /> </mets>

  22. <fileSec><fileGrp> <file ID="FID1" SIZE="184302" ADMID="TMD1PREMIS TMD1MIX DP1EVENT DP1AGENT“ CHECKSUM="4638bc65c5b9715557d09ad373eefd147382ecbf" CHECKSUMTYPE="SHA-1"> <FLocat LOCTYPE="OTHER" xlink:href="BXF22.JPG" /> </file></fileGrp></fileSec> <techMD ID="TMD1PREMIS"> <mdWrap MDTYPE="PREMIS"> <xmlData><premis:object > <objectCharacteristics> <fixity> <messageDigestAlgorithm>SHA-1 </messageDigestAlgorithm> <messageDigest>4638bc65c5b9715557d09ad373eefd147382ecbf  </messageDigest> <messageDigestOriginator>EchoDep/messageDigestOriginator> </fixity> <size>184302</size> </objectCharacteristics> Elements defined in both METS and PREMIS: METS: Checksum, Checksumtype attribute of <file> not repeatable PREMIS: fixity also includes messageDigestOriginator allows multiples

  23. <fileSec><fileGrp> <file ID="FID1" ADMID="TMD1PREMIS DP1EVENT DP1AGENT“ MIMETYPE="image/jpeg" <FLocat LOCTYPE="OTHER" xlink:href="BXF22.JPG"/> </file></fileGrp></fileSec> <techMD ID="TMD1PREMIS“ <mdWrap MDTYPE="PREMIS"> <xmlData> <premis:object> <objectCharacteristics> <format> <formatDesignation> <formatName>image/jpeg</formatName>   <formatVersion>1.02 </formatVersion> </formatDesignation></format> </objectCharacteristics> Elements defined both in METS and PREMIS: METS: MIMETYPE attribute of <file> optional PREMIS: <format> more granular; includes name and version (although name may be MIMETYPE) mandatory

  24. <fileSec> <fileGrp> <file ID="FID1" ADMID="TMD1PREMIS TMD1MIX DP1EVENT DP1AGENT"> <techMD ID="TMD1PREMIS"> <linkingEventIdentifier> <linkingEventIdentifierType>ECHODEP Hub Event </linkingEventIdentifierType> <linkingEventIdentifierValue>echo12345</linkingEventIdentifierValue> </linkingEventIdentifier> <digiprovMD ID="DP1EVENT"> <premis:event> <eventIdentifier> <eventIdentifierType>ECHODEP Hub Event</eventIdentifierType> <eventIdentifierValue>echo12345 </eventIdentifierValue> </eventIdentifier> <eventType>ingestion</eventType> <eventDateTime>2006-05-02T15:12:53 </eventDateTime></event> Elements defined both in METS and PREMIS METS ID/Idref: used to associate metadata in different sections and for different files PREMIS identifiers: explicit linking between entity types

  25. <structMap TYPE=“physical”> <div ORDER="1" TYPE="text"> <:fptr FILEID="FID9"/> <div ORDER="1" TYPE="page" LABEL=" Page [1]"> <fptr FILEID="FID1"/></mets:div> <div ORDER="2" TYPE="page" LABEL=" Page [2]"> <fptr FILEID="FID2"/></mets:div> </div> <relationship> <relationshipType>structural</relationshipType> <relationshipSubType>is sibling of </relationshipSubType> <relatedObjectIdentification> <relatedObjectIdentifierType>UCB</relatedObjectIdentifierType> <relatedObjectIdentifierValue>FID2</relatedObjectIdentifierValue> <relatedObjectSequence>1</relatedObjectSequence> Elements defined both in METS and PREMIS: METS: structMap details structural relationships and is the heart of the METS document hierarchical, so may be more expressive than PREMIS semantic units links the elements of the structure to content files and metadata PREMIS: <relationship> details all kinds of relationships, including structural data dictionary says that implementations may record by other means

  26. How to record elements from 2 different technical metadata schemas • Format specific metadata may be included in addition to PREMIS general technical metadata • Use multiple techMD sections and specify source in MDType attribute and/or namespace declaration • e.g. MDTYPE=“NISOIMG” or “PREMIS” • Give MIX schema declaration in METS document • MIX was recently revised to correspond with the revision of the Z39.87 technical metadata for digital still images standard; names harmonized with corresponding PREMIS semantic units • For digital still images, best practice may be to use PREMIS for general semantic units defined in PREMIS and MIX for format specific units without redundancy

  27. MPEG-21 Digital Item Declaration (DID) • ISO/IEC 21000-2: Digital Item Declaration • A promising alternative to represent Digital Objects • Starting to get supported by some repositories, e.g., aDORe, DSpace, Fedora • A flexible and expressive model that easily represents compound objects (recursive “item”) • Attach well-formed XML from persistent namespaces as metadata • Strong industry support

  28. Abstract Model for MPEG-21 DID container: grouping of items and descriptor/statement constructs pertaining to the container container item: represents a Digital Item aka Digital Object aka asset. Descriptor/statement constructs convey information about the Digital Item descriptor/statement item component: binding of descriptor/statements to datastreams descriptor/statement item resource: datastream descriptor/statement component component descriptor/statement resource resource resource

  29. Implementing PREMIS in DID • DID abstract model is an object-centric containment model • Semantically, Descriptor/statement constructs under a certain level are the metadata “about” that level of DID container or item or component. • Descriptor/statement about the DID container should be mapped to OAIS packaging information, therefore out of the PREMIS scope • Rights, Agents, and Events in the PREMIS model are linked to the objects, but not about the objects. • However, the PREMIS metadata as a whole (premis:premis), is about an object (the target of the preservation)

  30. Mapping All rights, events, and agents go here. The top level object goes here. Other objects may be duplicated here or linked here. DID DIDInfo object1 premis:premis object2 premis:object object3 object4 premis: object premis: object resource resource resource

  31. Partial Implementation in DID When metadata are not sufficient to form the top level PREMIS elements, partial implementation may be done if PREMIS elements are globally defined. DID DIDInfo object1 premis:premis object2 premis:significantProperties object3 object4 premis: creatingApplication premis: format resource resource resource

  32. Examples of PREMIS in XML containers • PREMIS in METS: • Portrait of Louis Armstrong(Library of Congress) • PREMIS in MPEG DID: • aDORe example (LANL)

  33. Proposed schema changes for new version • Define an abstract object type to allow for better validation of object category (representation, file, bitstream) • Define elements and types globally to allow for reuse • Implement an extensibility mechanism to provide for further structure when needed • Implement a mechanism to use controlled vocabularies • Adjust schemas to support changes in version 2 of data dictionary

  34. Summary: container formats • A container format is needed to package together all forms of metadata (of which PREMIS is one) and digital content • Use of a container is compatible with and an implementation of the OAIS information package concept • Co-existence with other types of metadata requires best practices for both approaches; redundancy seems to be preferred • Changes to the next version of the PREMIS XML schemas will facilitate a phased approach to full PREMIS implementation • Development of registries (informal or formal) for controlled vocabularies will benefit implementation

  35. Summary: METS vs. MPEG 21 DID • METS and MPEG DID are similar types of container formats in that both are expressed in XML, both represent the structure of digital objects, and both include metadata • MPEG DID doesn’t have the segmentation in metadata sections that METS does, so this implementation decision need not be made in DID • METS is open source and developed by open discussion, mainly cultural heritage community • MPEG DID is an ISO standard and has industry support, but is often implemented in a proprietary way and standards development is closed • It would be possible to transform a METS container to a MPEG DID and vice versa; development of stylesheets will enable transformations

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